Re: [PATCH 02/10] tick/nohz: Add tick_nohz_full_this_cpu()

[Peter Zijlstra]

On Thu, Mar 11, 2021 at 01:37:00PM +0100, Frederic Weisbecker wrote:
> Optimize further the check for local full dynticks CPU. Testing directly
> tick_nohz_full_cpu(smp_processor_id()) is suboptimal because the
> compiler first fetches the CPU number and only then processes the
> static key.
> 
> It's best to evaluate the static branch before anything.

Or you do tricky things like this ;-)

diff --git a/include/linux/tick.h b/include/linux/tick.h
index 7340613c7eff..bd4a6b055b80 100644
--- a/include/linux/tick.h
+++ b/include/linux/tick.h
@@ -185,13 +185,12 @@ static inline bool tick_nohz_full_enabled(void)
 	return tick_nohz_full_running;
 }
 
-static inline bool tick_nohz_full_cpu(int cpu)
-{
-	if (!tick_nohz_full_enabled())
-		return false;
-
-	return cpumask_test_cpu(cpu, tick_nohz_full_mask);
-}
+#define tick_nohz_full_cpu(_cpu) ({					\
+	bool __ret = false;						\
+	if (tick_nohz_full_enabled())					\
+		__ret = cpumask_test_cpu((_cpu), tick_nohz_full_mask);	\
+	__ret;								\
+})
 
 static inline void tick_nohz_full_add_cpus_to(struct cpumask *mask)
 {